1. Field of the Invention
The present invention relates generally to the magnetic resonance art and, in particular, to a method for compensating for a phase error of a phase encoding gradient pulse in a fast spin echo (FSE) imaging technique.
2. Description of the Related Art
According to an FSE imaging method, different phase encoding gradients are applied to acquire the position information from an echo signal generated from a plurality of radio frequency (RF) pulses so as to obtain data corresponding to each line on a k-space and thereby construct an image. The FSE imaging method has an advantage of decreasing the imaging time to 1/4, 1/8 or shorter as compared to a conventional spin echo (SE) imaging method without image quality degradation. Accordingly, clinical attention has been focused on the FSE imaging technique.
However, in the FSE imaging technique of constructing an image from data obtained by applying different phase encoding gradients at different times, which corresponds to each line on a k-space, a constructed image may have a ringing artifact or a blurring due to a phase error when a phase encoding gradient is not linearly increased. Accordingly, a signal-to-noise ratio (SNR) or a degree of contrast may be decreased, which in turns degrades the quality of the image.
FIG. 1A illustrates pulse sequences for a conventional FSE imaging method. Corresponding to a phase encoding gradient pulse sequence shown in FIG. 1A, a k-space is filled with data obtained using phase encoding gradients, as shown in FIG. 1B. In FIG. 1B, the Kx-axis indicates a frequency encoding direction, and the Ky-axis indicates a phase encoding direction. In FIG. 1, reference numerals 1 through 8 denote gradient pulses used for application of phases. The eight gradient pulses have different amplitude. To recover the phases subjected to the gradient pulses 1 through 8 to original states, gradient pulses 11 through 18 are applied. Each gradient pulse 11 through 18 has the same amplitude as a corresponding gradient pulse 1 through 8, respectively, but with a different polarity. Therefore, the phases at positions 21 through 28 are the same.
However, it frequently happens that actually, the phases at positions 21 through 28 are not the same due to a variety of causes. In this case, a phase error occurs in the k-space. In a magnetic resonance imaging (MRI) apparatus, phase errors due to phase encoding gradients occur depending on a gradient system. Gradient offset, non-linearity of a gradient and eddy current are related to occurrence of a phase error. In particular, eddy current is fatal to the FSE imaging technique. Accordingly, an active shielded gradient is used to fundamentally prevent a magnetic field from changing due to eddy current. However, when this technique is used, the price of a gradient system becomes significantly increased, while the space for a patient in an MRI apparatus is decreased.
Meanwhile, in low-field MRI, a method of pre-emphasized gradient waveform using an eddy current compensation circuit is employed. However, since this method does not fundamentally remove eddy current, an operation of adjusting the amplitude of a gradient pulse is required. Moreover, this method cannot remove a phase error due to an external factor. Consequently, a compensation operation with respect to a phase encoding gradient pulse sequence is required to remove a phase error.
In an attempt to address the above problems, it is a feature of the present invention to provide a method of compensating for a phase error of a phase encoding gradient pulse in a fast spin echo (FSE) imaging method to improve the quality of an image.
Accordingly, in a preferred embodiment of the present invention, there is provided a method of compensating for a phase error of a phase pulse in fast spin echo imaging. The method includes the steps of (a) generating phase encoding gradient pulses for tuning encoding gradient between a 90xc2x0 RF signal and a first 180xc2x0 RF signal, the phase encoding gradient pulses for tuning having opposite polarities to each other and the same amplitudes as those of respectively phase encoding gradient pulses corresponding to the first 180xc2x0 RF signal, (b) adjusting the amplitude of either of the phase encoding gradient pulses for tuning such that a phase before the phase encoding gradient pulses for tuning is the same as a phase behind the phase encoding gradient pulses for tuning and obtaining the adjusted amplitude, and (c) compensating for the phase errors of the phase encoding gradient pulses corresponding to the first 180xc2x0 RF signal using the adjusted amplitude.
The method preferably further includes the step of (d) sequentially changing the amplitudes of the phase encoding gradient pulses for tuning to the amplitudes of phase encoding gradient pulses corresponding to second through n-th (n is an integer greater than 1) 180xc2x0 RF signals and repeating the steps (b) and (c) at each change to compensate for the phase errors of the phase encoding gradient pulses corresponding to the second through n-th 180xc2x0 RF signals, after the step (c).
In another preferred embodiment of the present invention, there is provided a method of compensating for a phase error of a phase encoding gradient pulse in fast spin echo imaging. The method includes the steps of (a) embedding a 180xc2x0 RF signal between a 90xc2x0 RF signal and a first 180xc2x0 RF signal and generating a selection gradient pulse and a frequency encoding gradient pulse, which correspond to the embedded 180xc2x0 RF signal, (b) generating phase encoding gradient pulses for tuning between the embedded 180xc2x0 RF signal and the first 180xc2x0 RF signal, each of the phase encoding gradient pulses for tuning having pulse elements which have polarities that are opposite to each other and the same amplitudes as those of respective phase encoding gradient pulses corresponding to the first 180xc2x0 RF signal, (c) adjusting the amplitude of either of the phase encoding gradient pulses for tuning such that a phase before the phase encoding gradient pulses for tuning is the same as a phase behind the phase encoding gradient pulses for tuning and obtaining the adjusted amplitude, and (d) compensating for the phase error of the phase encoding gradient pulse corresponding to the first 180xc2x0 RF signal using the adjusted amplitude.
The method preferably further includes the step of (e) sequentially changing the amplitudes of the phase encoding gradient pulses for tuning to the amplitudes of phase encoding gradient pulses corresponding to second through n-th (n is an integer greater than 1) 180xc2x0 RF signals and repeating the steps (c) and (d) at each change to compensate for the phase errors of the phase encoding gradient pulses corresponding to the second through n-th 180xc2x0 RF signals.